DEVELOPMENTAL - BIOLOGY (EMBRYOLOGY)
- Embryology is the branch of biology which involves the study of all those processes, which take place during the development of an adult from the egg.
- In lower animals, as a result of asexual reproduction the structure formed is termed as Blastema and it gives rise to the complete animal. This is termed as "Blastogenesis" and the animals so formed are termed as Blastozoids.
- In majority of animals, as a result of sexual reproduction the structure formed is termed as the Zygote and it gives rise to complete animal. This is termed as "Embryogenesis" and the animals, so formed are termed as Oozoids. They have the most advanced characters.
FORMATION OF GAMETES
Follicle stimulating hormone stimulates gametogenesis. Besides this hormone vitamin E is also essential for gametogenesis. Deficiency of vitamin E leads to sterility. Vitamin A is also required for the formation of healthy gametes.
Gametogenesis is divided in three stages:
(i) Multiplication phase
(ii) Growth phase
(iii) Maturation phase
As there are two types of gametes, the spermatozoa and ova, gametogenesis can be studied under two broad headings:
- Spermatogenesis is the formation of spermatozoa, whereas oogenesis is the formation of ova.
- Both spermatozoa and ova originate from primordial germ cells or PGCs, which are extra-gonadal in origin. In humans, the PGCs originate during early embryonic development from the extra-embryonic mesoderm.
- Eventually, they migrate to the yolk sac endoderm, and ultimately, to the gonads of the developing embryo, where they undergo further development. You can recall that spermatogenesis occurs in the seminiferous tubules of the testes and oogenesis occurs in the follicles of ovary. Formation of gametes starts at puberty.
Spermatogenesis i.e. formation of sperms. In most of the animals spermatogenesis takes place in testes, (exception-earthworm). Mammalian testes contain seminiferous tubules and wall of seminiferous tubule is composed of germinal epithelium.
- It contains some special types of cells called primordial germ cells and these cells start spermatogenesis. On the basis of origin, primordial germ cells are extra embryonic mesodermal.
- Besides these cells, germinal epithelium contains some large sized cell called Sertoli cells. Occurrence of sertoli cells is the unique feature of mammalian testis.
- Sertoli cells provide nutrition to developing sperm i.e. developing sperms are embedded in cytoplasm of sertoli cells and absorb nutrition. After maturation sperms comes out from sertoli cells and liberate in seminiferous tubules.
- Liberation of sperms from Sertoli cells is called Spermination.
- Liberation of sperms from testes is called Semination.
- Liberation of sperms from body of male is called Ejaculation.
- Mammalian sperms are transferred to vagina of female by the process called insemination.
- Sertoli cells form 'Blood testes barrier' and protect the sperm from immune system of the body. (because antibody may attack on haploid cells and destroy them. (Sperms are haploid and other cells of body are diploid).
Sertoli cells function as an endocrine gland. i.e. secrete three types of hormones:
(i) Antimullerian hormone: Function of this hormone is degradation of female gonads in male embryo (In male seminal vesicle is the ruminant part of oviduct of female).
(ii) Inhibin hormone: Function of this hormone is to control excess secretion of pituitary gland and prevent the over-production of sperms.
(iii) Androgen binding protein: Function of this hormone is to concentrate testesterone in seminiferous tubules because testosterone is must for spermatogenesis in seminiferous tubules.
Spermatozoa are formed in the wall of the seminiferous tubules of the testes.The various cell-stages in spermatogenesis are as follows ( the number of chromosomes at each stage is given in brackets).
An adult male produces over 1012 to 1013 sperm cells each day. These gradually move into the epididymis and the first portion of the vas deferens, where they undergo further maturation and are stored.
Fig: Spermatogonia development
- The spermatogonia ( type A) or germ cells (44 X+Y) divide mitotically, to give rise to more spermatogonia of type A (spermatogenic lineage) and also spermatogonia of type B.
- The spermatogonia (type B) (44 + X + Y) enlarge, to form primary spermatocytes.
- The primary spermatocytes (44 + X + Y) now divide so that each of them forms two secondary spermatocytes. This is the first meiotic division: it reduces the number of chromosomes to half.
- Each secondary spermatocyte has 22 + X or 22 + Y chromosomes. It divides to form two spermatids. This is the second meiotic division and this time there is no reduction in chromosome number(spermatocytogenesis).
- Each spermatid (22 + X or 22+Y) gradually changes its shape to become a spermatozoon.This process of transformation of a circular spermatid to a spermatozoon is called spermiogenesis/spermateleosis.
In spermiogenesis first, of all nucleus of spermatid shift at one side. Except chromatin material, all the structures come out from nucleus as a result nucleus become small and light in weight. Then several golgi vesicles are gathered just above the nucleus.
Some of the Golgi vesicles develop granules called Proacrosomal granules, such granule containing vesicle is called Proacroblast. Rest of the golgi vesicles are called Golgirest, the golgirest dissolve in cytoplasm. All the proacroblast fuse and form large vesicle called Acroblast and it's granule is called Acrosomal granule.
Acroblast arranged just above the nucleus. Then cytoplasm of spermatid starts moving towards posterior side. As a result, plasma membrane shrinks and gets attached to acroblast and nucleus. Due to this head of sperm is formed. Now centriole of spermatid are arranged at right angle to each other just beneath the nucleus and distal centriole starts forming axonema, then all the mitochondria of spermatid are arranged in spiral order around axonema. In this way middle piece of sperm is formed. Now axonema elongates towards posterior side and forms tail.
Cytoplasm of spermatid continuously flows towards posterior side. At last cytoplasm constrict from base and detach from sperm in from of large mass and mass of cytoplasm received by sertoli cells through method of phagocytosis and provide nutrition to sperm.
Structure of sperm:
Fig: Sperm structure
- The spermatozoon has a head, a middle piece and a tail. The head is covered by a cap called the Acrosomic cap, Anterior nuclear cap,or Galea capitis.
- Acrosome is a bag like structure filled with lytic enzymes called Sperm lysins.
- In the anterior part of middle piece neck is present. The neck is narrow : it contains a proximal & distal centriole (or Basal body).
- An axial filament begins just behind this centriole, it passes through the middle piece and extends into the tail. At the point where the middle piece joins the tail, this axial filament passes through a ring-like structure called the Annulus (or ring centriole or zensons ring).
- That part of the axial filament which lies in the middle piece, is surrounded by a spiral sheath made up of mitochondria. (Nebenkern sheath) Nuclear part of head of spermatozoa consist of chromatin (mostly DNA) that is extremely condensed. It contains a basic nature protein called Protamin.
The axial filament, that passes through the middle piece and most of the tail, is actually composed of several fibrils arranged. There is a pair of central fibrils, surrounded by nine pairs (doublets) arranged in a circle around the central pair (9+2). This arrangement of one central pair of fibrils surrounded by nine doublets is covered by nine solid protein fibres in the middle piece (making the arrangement as 9 + 9 + 2).
In the proximal tail part it is covered by only two solid protein fibres (therefore arrangement is 2 + 9 + 2), while end part of tail has no protein covering (therefore arrangement is 9 + 2). Immediately outside the fibrils there is a fibrous sheath.
Middle piece (also called as the energy chamber) is surrounded by spirally arranged mitochondria (Nebenkern sheath). Finally, the entire sperm is enclosed in a plasma membrane.
“Oogenesis is the process of formation of female gametes.”
- In females, the immature ovum is converted into the mature ovum. This process is called Oogenesis.
- In the female ovary, millions of mother cells are formed during fetal development. These mother cells undergo the meiotic cell division and lead to the production of primary Oocytes.
- Primary Oocytes are embedded with primary follicles on the outer layer.
- Primary follicles are bounded with more granulosa cell layer and form secondary follicles.
- Secondary follicles then turn into the tertiary follicle.
- At the stage of female puberty, tertiary follicles undergo some structural and functional changes and produce mature Graafian follicle.
- Ovum is released from the Graafian follicle during the menstrual cycle. The release of an ovum from the Graafian follicle is called Ovulation.
- Ovulation is controlled by the female reproductive hormone which is stimulated by the pituitary gland.
Difference Between Spermatogenesis And Oogenesis